The present invention relates to positioning systems such as Global Navigation Satellite Systems (GNSS) and, more particularly, to a method of mitigating multipath noise in such systems.
A GNSS system is a space-based global navigation system in which the satellites of a constellation of satellites in orbit around the Earth transmit signals that are used by terrestrial receivers for purposes such as navigation. The existing GNSS systems include the GPS system operated by the United States of America and the GLONASS system operated by Russia. The present invention is described herein in terms of the GPS system. It will be clear to those skilled in the art how to use the principles of the present invention in the context of other GNSS systems such as the forthcoming European Galileo system or the forthcoming Chinese Compass navigation system.
The GPS system includes between 24 and 32 satellites in medium Earth orbit. The GPS ranging code that is available for public use is the C/A code which is a PseudoRandom binary Code (PRN) of 1,023 bits. Each satellite continuously transmits, as its ranging signal, sequences of its own unique PRN code that is orthogonal to all the other PRN codes. Modulated on top of each satellite's C/A code is a navigation message that includes an ephemeris (plural: ephemerides) that describes the satellite's orbit. The ephemerides are updated every two hours.
A GPS receiver generates its own copies of the C/A codes, nominally synchronized with the satellite transmissions. When the receiver receives the C/A codes of the currently visible satellites (typically 12-16 satellites in open terrain), the receiver cross-correlates its copies of the C/A code with the received C/A codes to determine the one-way travel times from the satellites to the receiver. Knowing the ephemerides, the receiver computes the locations of the satellites at the times of the transmissions to obtain ranges to the satellites. If the receiver's clock were perfectly synchronized with the satellites then signals from three satellites would suffice to triangulate the location (horizontal coordinates x and y and elevation z) of the receiver. Because clocks that could be synchronized that well with the atomic clocks used by the satellites are far too expensive for routine use, signals from four or more satellites are used to determine x, y, z and the time offset dt of the receiver's clock from the GPS clocks. If only the horizontal coordinates are needed for navigation then signals from three satellites suffice.
Dense urban environments present some of the hardest challenges for GNSS receivers. The characteristics of these environments, also known as “urban canyons”, generate two negative effects on GNSS receiver performance:                Line of sight blockage between a transmitting satellite and the GNSS receiver due to high buildings decreasing the number of visible satellites.        Erroneous measurements (multipath errors) caused by satellite transmissions that are reflected from adjacent buildings and received by the GNSS receiver.        
It would be highly advantageous to have a GNSS receiver that is more resistant to multipath noise than presently known GNSS receivers.